Add RelativeLinkFilter section to Markdown fixture

spec/fixtures/markdown.md.erb

@@ -100,6 +100,13 @@ Markdown should be usable inside a link. Let's try!
 - [**text**](#link-strong)
 - [`text`](#link-code)
+### RelativeLinkFilter
+
+Linking to a file relative to this project's repository should work.
+
+[Relative Link](doc/README.md)
+![Relative Image](app/assets/images/touch-icon-ipad.png)
+
 ### EmojiFilter
 Because life would be :zzz: without Emoji, right? :rocket: